pinctrl: single: fix potential NULL dereference

Added checking of pointer "function" in pcs_set_mux().
pinmux_generic_get_function() can return NULL and the pointer
"function" was dereferenced without checking against NULL.

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Fixes: 571aec4d

 ("pinctrl: single: Use generic pinmux helpers for managing functions")
Signed-off-by: Maxim Korotkov <korotkov.maxim.s@gmail.com>
Reviewed-by: Tony Lindgren <tony@atomide.com>
Link: https://lore.kernel.org/r/20221118104332.943-1-korotkov.maxim.s@gmail.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>